Various industrial processes produce vapors and gases. These vapors and gases should be treated to avoid release of pollutants into the atmosphere. Other industries produce by-product vapors and gases, e.g., manufacturing of paint or polyethylene. Recapture of these by-products increases the yield of a manufacturing process. Various processes have been developed to treat vapor and gas streams to serve these and other applications. Each seeks to remove vapors and gases from a gas stream.
Adsorption is a particularly useful technique. Adsorption removes a wide range of gas stream components. Adsorption process typically includes an adsorption step and a desorption step. During the adsorption step, the gas stream is brought into contact with sorbent such as activated carbon or zeolites. Vapors and gases adhere to the sorbent carbon or zeolite due to molecular attractive forces. The adsorbed vapors and gases (sorbate) are released from the adsorbent during the desorption step. They are typically released by lowering pressure or by raising temperature. A typical method to raise temperature is by injection of steam. In the conventional processes, released adsorbed gases are often burned or converted to liquid through a condensation system that is located downstream of the desorption flow from an adsorption/desorption system.
Other emission control techniques include material substitution, thermal oxidation, membrane filtering, biofiltration, and absorption. The adsorption technique is advantageous due to its wide ranging applicability, comparatively low energy requirements, and its ability to recover gas stream components which have been recaptured. Adsorption also offers selectivity. Selectivity results if the temperature in the adsorption/desorption system can be controlled during the adsorption step because different vapors and gases will adsorb to the sorbent carbon or zeolite at different temperatures. Selectivity has become better controlled with the use of activated carbon fiber cloth (ACFC) as a sorbent, which has also yielded additional improvements to adsorption. ACFC has approximately twice the capacity of conventional adsorbents. It permits the rapid capture of volatile organic contaminants even when the contaminants have low concentrations in the gas stream. ACFC is ash free, which inhibits reactions with vapors such as ketone containing organic compounds and alkenes. Many ACFC processes follow the model of the above described adsorption/desorption processes, with ACFC taking the place of beds of zeolites or granular carbon.
Adsorption/desorption systems provide for the collection and subsequent treatment of adsorbate. Biofilters and oxidizers are typically used for destruction of the sorbate from high flow rate gas streams. Biofilters and oxidizers need to be designed to tolerate the swings in concentration in the sorbate concentration within gas streams. The input fluctuations in adsorbed vapors and gases result in over-designing of biofilters and oxidizers. Use of an adsorption/desorption system upstream of these devices allows for the gas stream to be treated by the biofilter or oxidizer at a much lower flow rate and at a constant concentration specifically designed for that downstream device.
U.S. Pat. No. 6,364,936 to Rood et al. advanced the state of the art for AFCF processes. The '936 describes devices and method that provide for the selective sorption and desorption of vapors and gases with electrically heated activated carbon fiber cloth elements. Sorbate output by the devices in the '936 patent is either in liquid or gas phase. Devices described in the '936 patent include an adsorption/desorption system having one or more elongated hollow ACFC elements. The geometric configuration of the ACFC element or elements is designed such that the elements have an electrical resistance value sufficient to permit heating of the elements by electrical current to a temperature that permits adsorption of a select gas stream constituent or constituents. The geometry also permits gas flow to penetrate the ACFC element(s). An enclosure houses the ACFC elements and is arranged to direct a gas stream through the elements and into and out of the enclosure via gas ports. The ability to heat the elements to a desired temperature by electrical current allows for straightforward implementation of selective adsorption. After an adsorption step, altering the temperature of the element or elements further enables desorption.